The existing art has disclosed textile planar structures that are formed from two or more textile plies, arranged one above another and constituted as fabric plies, that fundamentally represent independent woven structures. It is characteristic of these that each fabric ply has intersecting mutually interwoven structural yarns, i.e. first structural yarns, for example warp yarns, and second structural yarns transversely thereto, for example weft yarns. Fabrics of this kind are used, in particular, as forming fabric in the sheet-forming region of a papermaking machine. They are theoretically also suitable, however, for being provided in other sections of a papermaking machine if they are correspondingly adapted or additionally equipped with fiber plies, for example in order to form a fiber felt. They are also suitable, for example, as transport belts or filtering means.
Engineering fabrics made up of two or more independent fabric plies create the possibility of adapting the fabric plies to the particular requirements by selecting the nature, number, thickness, and material of the structural yarns. For example, when such fabrics are used in the papermaking machine sector it is common to manufacture the fabric ply that is intended to support the paper web from fine structural yarns having a weave pattern such that good fiber and filler retention is achieved and marking of the paper web, which is still very sensitive in this region, is prevented, but at the same time so that dewatering is also not substantially impeded. For the machine-side fabric ply it is usual to use a smaller number of structural yarns that have a larger diameter, in order to ensure good abrasion resistance and dimensional stability for the overall structure, i.e. to prevent longitudinal extensions and/or transverse shrinkage under load. Fine-yarn and coarse-yarn fabric plies of this kind can also be of multiple-ply configuration.
A problem that exists with such engineering fabrics, also called composite fabrics, is that of joining the fabric plies to one another. Two fundamentally different joining techniques have been developed in this context.
In the first joining technique, additional binding yarns that bind into two adjacent fabric plies are used. They do not belong the regular fabric weave of either the one fabric ply or the other fabric ply, i.e. do not constitute structural yarns. The binding yarns can run in either the warp or the weft direction (cf. U.S. Pat. No. 4,987,929; U.S. Pat. No. 5,518,042; U.S. Pat. No. 5,709,250; EP-B-0 579 818; U.S. Pat. No. 4,815,499; U.S. Pat. No. 4,729,412, FIG. 1). DE-A-42 29 828 and EP-A-0 408 849 also depict and describe binding yarns running in one direction; EP-A-0 408 849 showing a paired arrangement of two binding yarns in each case, which respectively alternate in the fabric plies that are joined by them. Casual mention is made of the possibility of providing binding yarns in both the longitudinal and the transverse direction, but such an arrangement is not explained or shown in further detail. Intersecting binding yarns of this kind are, however, explicitly evident from DE-A-34 11 119 and DE-C-33 01 810. In both cases, the binding yarns join the fabric plies not directly, but indirectly by forming an elastic intermediate layer, between the fabric plies, that is made up exclusively of the two binding yarn systems.
The joining technique described above has the disadvantage that yarns foreign to the structure are woven into the fabric as binding yarns. They engage irregularly into the binding weave and disrupt its uniformity, even if they are arranged respectively in pairs (cf. U.S. Pat. No. 4,987,929; U.S. Pat. No. 5,518,042; U.S. Pat. No. 5,709,250; EPA-0 408 849). This results in inhomogeneities in water removal and markings due to denting (dimpling effect) in the paper-side surface. In order to minimize these effects, relatively thin binding yarns are used. But because the binding yarns are subjected to large forces and moreover to abrasion due to mutual displacement of the fabric plies, a compromise must be found in this regard. This also applies to the number of binding yarns, since too large a number of such yarns would interfere with dewatering.
With the second type of joining technique, the structural yarns of at least one fabric ply are employed to join the fabric plies. These are not additional yarns, but those that are an integral component of the respective fabric ply. Examples of this may be seen in U.S. Pat. No. 4,605,585, U.S. Pat. No. 5,244,543, U.S. Pat. No. 5,564,475, EP-B-0 224 276, U.S. Pat. No. 4,501,303, U.S. Pat. No. Re.35,777, and EP-A-0 794 283. In the four first-named documents, all the structural longitudinal yarns of the paper-carrying fabric ply bind into the ply located therebelow, in some cases in such a way that each two adjacent structural yarns in the paper-carrying fabric ply alternate (cf. U.S. Pat. No. 4,605,585; EP-B-0 224 276). In the fabric according to U.S. Pat. No. Re.35,777, the binding structural yarns run in the transverse direction.
The three last-named documents above describe fabrics in which only a portion of the structural yarns running in one direction form binding structural yarns, by the fact that they bind not only into the paper-carrying fabric ply but also into the machine-side fabric ply. In this context, two binding structural yarns run next to each other in each case, i.e. form a pair of structural yarns, the manner in which they bind in being such that they alternate in the two fabric plies, i.e. when the one binding structural yarn is binding into the first fabric ply, the second binding structural yarn is binding into the other fabric ply. The two binding structural yarns thus intersect within the fabric. The binding-in within the respective fabric ply is such that the portions of the pairs of binding structural yarns and non-joining structural yarns that bind thereinto yield a desired weave pattern.
This joining technique also has disadvantages. If too many or indeed all of the-structural yarns of a fabric layer are bound in as binding structural yarns, the result is a very uneven surface, at least on the outer side of that fabric ply. If only a few structural yarns are employed as binding structural yarns, the joining of the fabric layers is not strong enough, so that relative movements occur between the fabric plies. This in turn results in internal friction, which causes premature wear with the risk of delamination. In addition, the structural binding yarns are then so highly stressed in tension that here again denting results, with the risk that marking in the paper web may occur.